Many military radars, such as Aegis, use phased array antennas. The elements of phased arrays must be spaced apart a distance less than .lambda./2, where .lambda. is the highest frequency for which the antenna will be used. At higher frequencies, the spacing between elements becomes so small that the effective power of the radar becomes unacceptably small, and the cost to fabricate the array becomes prohibitively large. Also, the agility of such a phased array, i.e. the speed with which it can switch among its various "look angles," depends on the delay times needed to properly phase the array.
As is well known, a plasma reflects electromagnetic radiation whose frequency .nu. is less than the plasma frequency .nu..sub.p =9(10).sup.3 n.sub.e.sup.1/2, where n.sub.e is the plasma electron density in cm.sup.-3 and .nu..sub.p is in Hz. It has been suggested that this property could be exploited to make plasma mirrors which could take the place of the usual metallic reflectors in some radar systems. See W. M. Manheimer, "Plasma Reflectors for Electronic Beam Steering in Radar Systems," IEEE Trans. Plasma Sci., vol. PS19, p. 1228 (1991); see also, NRL Memorandum Report 6809 (Washington, D.C., Naval Research Laboratory, April, 1991) a document of the same author and title. Such plasma mirrors would be virtually inertialess and would allow the radar to be redirected on a timescale determined by the time that it takes for a suitable plasma to be created and to decay. This could lead to radar systems having the directional agility of phased arrays, but with greatly reduced complexity and cost. Furthermore, the plasma mirror could be used at higher frequencies than those currently considered practicable for phased arrays.